1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrographic copying machine, a printer and a facsimile machine of a type in which after an electrostatic latent image is formed on a charged surface of an electrostatic latent image carrier such as a photoreceptor drum, the electrostatic latent image is developed into a toner image.
2. Description of the Prior Art
Typically, an electrographic copying machine is provided with, for example, a photoreceptor drum rotating at a constant speed, and along the rotation direction of the drum, a charging section, an exposing section, a developing section, a transferring section, a cleaning section and a charge removing section are arranged. The copying operation is performed in the following manner: First, the drum surface is charged at the charging section, then, the drum rotates, and when the charged drum surface passes the exposing section, light carrying the image information of the original exposes the drum surface. By the exposure light, an electrostatic latent image is formed on the drum surface.
In the developing section, a developer unit is arranged to face the drum surface. When the drum rotates to reach the developing section, toner supplied from the developer unit adheres onto the electrostatic latent image formed on the drum surface, thereby obtaining a toner image. At the transferring section, the toner image is transferred onto the surface of a copy sheet supplied from a paper feeding section. After the transfer is completed, the residual toner on the drum surface is removed at the cleaning section and the electrostatic latent image formed on the drum surface is removed by irradiating charge removing light onto the entire surface of the drum at the charge removing section to optically attenuate the surface potential.
In the electrographic copying machine structured as described above, a charger employing a corona discharge method is arranged to face the drum surface in the charging section. In this arrangement, when a copy button is depressed in the standby state of the copying machine, a high voltage of normally approximately 4 to 6 kV is applied to a discharging main wire of the charger to generate a corona discharge, thereby applying a charge to the drum surface. Conventionally, in order to supply the high voltage to the discharging main wire, a transformer board incorporating a transformer for generating a high voltage is provided between the main wire and the power source, and the output of the transformer board is controlled by a main circuit board so as to take a substantially constant value.
In recent years, amorphous silicon materials have been widely used as photosensitive materials for the above-described drum type or other types of electrostatic latent image carriers provided in image forming apparatuses of this type. As the result of experiments and examinations, the amorphous silicon materials are regarded as inferior in rise characteristic of the surface potential in charging unlike conventional arsenic selenium materials.
Specifically, it is considered that the rise condition of surface potential of the drum in charging is such that, as shown by a curve a.sub.1 of FIG. 12, it takes a long time for the surface potential to reach a stable potential after the depression of the copy button in the initial operation of the copying process and that, as shown by a curve b.sub.1 of FIG. 12, from a relationship with the length of a shelf time from the end of a copying process to the start of the next copying process, the longer the shelf time is, the more the rise of the drum surface potential deteriorates.
For this reason, it is considered that in electrostatic latent image carriers using amorphous silicon materials, in reducing the length of the first copying i.e. a copying operation performed for the first time in order to increase the copying efficiency, as shown in FIG. 12, the drum surface potential is still lower than the stable potential at a point of time t.sub.f when the first copying (copying of the first copy sheet) is performed, and that the rise of the surface potential further deteriorates when the shelf time from the end of a copying process to the start of the next copying process exceeds an hour.
To solve the problems of photoreceptor drums having such characteristics, in Japanese Patent Application H6-2557 directed particularly to amorphous silicon-made photoreceptors inferior in rise of the charging, the inventors of the present invention proposed to correct the control amount of the transformer board so that the charging stays flat until the surface potential stabilizes. Moreover, in Japanese Patent Application H6-2558 the present inventors proposed to vary the correction value according to the shelf time during which the drum is left deactivated.
However, the present inventors further carried out detailed experiments and verifications and found that not all the amorphous silicon materials are inferior in the above-described initial charging characteristic and shelf time characteristic. Specifically, some materials have a characteristic such that, as shown in a curve a.sub.2 of FIG. 12, in the initial operation of the copying process, the surface potential is rapidly activated in contrast to the conventional recognition and temporarily exceeds the stable potential to overshoot, and thereafter, the surface potential gradually returns to the stable potential and remains stable.
In the case of drums having such characteristics, it is confirmed that the shelf time characteristic is such that the longer the shelf time is, the more rapidly the drum surface potential rises as shown by a curve b.sub.2 of FIG. 12. In the case of drums having such a characteristic, the surface potential is higher than the stable potential in the copying of the first copy sheet. For this reason, the charge of the electrostatic latent image is excessive, so that the electrostatic latent image cannot be developed into an excellent toner image at the developing section.
Therefore, if continuous copying is performed with such an arrangement, since the surface potential is high during copying of several sheets after the start of the copying, obtaining copy images of desired quality is difficult until the completion of copying of the several sheets that the drum surface potential reaches the normal value. Moreover, only by the above-described solution of the prior art, the surface potential is over-corrected and the charging characteristic may become all the worse for the over-correction.